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Date:   Thu, 7 Nov 2019 15:37:05 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     Jan Stancek <jstancek@redhat.com>, linux-kernel@vger.kernel.org,
        ltp@lists.linux.it, viro@zeniv.linux.org.uk,
        kstewart@linuxfoundation.org, gregkh@linuxfoundation.org,
        rfontana@redhat.com
Subject: Re: [PATCH] kernel: use ktime_get_real_ts64() to calculate
 acct.ac_btime
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On Thu, 7 Nov 2019, Peter Zijlstra wrote:
> On Thu, Nov 07, 2019 at 01:40:47PM +0100, Thomas Gleixner wrote:
> > On Thu, 7 Nov 2019, Peter Zijlstra wrote:
> 
> > > +	mono = ktime_get_ns();
> > > +	real = ktime_get_real_ns();
> > > +	/*
> > > +	 * Compute btime by subtracting the elapsed time from the current
> > > +	 * CLOCK_REALTIME.
> > > +	 *
> > > +	 * XXX totally buggered, because it changes results across
> > > +	 * adjtime() calls and suspend/resume.
> > > +	 */
> > > +	delta = mono - tsk->start_time; // elapsed in ns
> > > +	btime = real - delta;		// real ns - elapsed ns
> > > +	do_div(btime, NSEC_PER_SEC);	// truncated to seconds
> > > +	stats->ac_btime = btime;
> > 
> > That has pretty much the same problem as just storing the CLOCK_REALTIME
> > start time at fork and additionally it is wreckaged vs. suspend resume.
> 
> It's wrecked in general. It also jumps around for any REALTIME
> adjustment.
> 
> > So a CLOCK_REALTIME time stamp at fork would at least be correct
> > vs. suspend resume.
> 
> But still wrecked vs REALTIME jumps, as in, when DST flips the clock
> back an hour, your timestamp is in the future.
> 
> Any which way around the whole thing is buggered.  The only real fix is
> not using REALTIME anything. Which is why I'm loath to add that REALTIME
> timestamp at fork(), it just encourages more use.

Fair enough. You have a sane alternative though: CLOCK_TAI

Thanks,

	tglx